The present invention relates to a device and a method for determining blood recirculation in a vascular access.
The efficiency of a dialysis treatment is defined as the ratio of the volume of blood purified from by-products of the metabolism (urea, creatinin, etc.) during the dialysis session to the patient's total blood volume.
A simplified model of the blood flows that occur when an extracorporeal blood circuit is connected to the vascular system of a patient through a corporeal access of the type of a Cimino-Brescia fistula is shown in FIG. 1, in which 1 denotes the heart, 2 denotes the pulmonary circuit, 3 denotes the vascular system (or systemic circuit) and 4 denotes a dialyzer, connected to the systemic circuit 3 via an inlet line 5 (arterial line) and an outlet line 6 (venous line).
As can be seen from FIG. 1, the blood treated in the course of a dialysis session comes from the systemic circuit 3, in which the blood flows at a limited flow rate; therefore existing dialysis treatments have an intrinsically rather low dialytic efficiency and at present there are no corrective measures by which its value can be increased.
Moreover, the efficiency of a dialysis treatment is further reduced by the phenomenon known in medical circles as recirculation in the vascular access: on account of a number of factors such as the flow rate of blood circulating in the extracorporeal circuit, the position of the needles and the degree of stenosis of the fistula, a portion of the blood circuit which is returned, after the dialysis treatment, into the patient's body via the venous line 6, immediately re-enters the extracorporeal circuit via the arterial line 5, as shown in the detail in FIG. 2. FIG. 2 represents a vascular access (fistula), in which the needles for withdrawal and re-admission of the blood are denoted by 7 and 8.
The value AR of recirculation in the vascular access is generally defined by the following equation:                                           A            R                    ⁢          %                =                                            Q              R                                      Q              B                                ·          100                                    (        1        )            in which QB is the blood flow circulating in the extracorporeal circuit and QR is the blood flow returning to the extracorporeal circuit via the arterial line 5 immediately after dialysis treatment.
Knowledge of the value AR of recirculation in the vascular access is of considerable importance in dialysis treatments for many reasons, such as the repositioning of the needles 7, 8 when the value AR of the recirculation becomes too high, increase in accuracy of dialysis treatment, long-term monitoring of the stenosis of the fistula and increase in average life of the fistula.
For determination of the value AR of recirculation in the vascular access, numerous methods of measurement are known, which can be placed in two broad groups, the first comprising non-provocative methods of measurement and the second comprising the provocative methods of measurement.
The first group includes methods of measurement that do not involve chemical or physical stressing of the blood undergoing dialysis treatment but are limited to quantifying physiological values in the course of the dialysis session.
For example, this first group includes the method of measurement “with urea samples”, which involves measuring the concentration of urea in three blood samples taken simultaneously in the arterial line, in the venous line and the patient's peripheral circuit, and calculating the value AR of recirculation in the vascular access according to the equation (equivalent to 1):                                           A            R                    ⁢          %                =                                                            C                S                            -                              C                A                                                                    C                S                            -                              C                T                                              ·          100                                    (        2        )            in which CS is the value of the urea concentration in the peripheral circulation (systemic concentration), CA is the value of the urea concentration in the arterial line (arterial concentration), and CV is the value of the urea concentration in the venous line (venous concentration).
However, this method has the drawback of being based on the basic assumption that, in the absence of recirculation in the vascular access, the value of the systemic concentration CS is equal to the value of the arterial concentration CA; it has recently been demonstrated, however, that this assumption is not valid in all conditions and depends on the collection point, therefore even in the absence of recirculation in the vascular access there are differences between these values, which prejudices the reliability of the measurement.
On the other hand, the second group includes methods of measurement involving chemical or physical stressing of the blood undergoing the dialysis treatment.
This second group includes for example the method of measurement “with urea samples and minimum QB” which is substantially identical to the method of measurement “with urea samples” described above, differing from it in that blood sampling in the arterial line for determining the value CS of the systemic concentration is effected in conditions of minimum circulating blood flow QB in the extracorporeal circuit, so as to minimize recirculation in the fistula and hence reduce the differences between the values CS and CA of systemic concentration and arterial concentration.
The second group also includes methods of measurement “in dilution” which envisage administration of a tracer to the patient, in order to obtain blood dilution of a chemical and physical character, and simultaneous monitoring, by means of special sensors, of its behaviour with respect to one or both of the arterial and venous lines. By comparing the signals detected by the sensors it is possible to determine, in a known manner which is therefore not described in detail, the value AR of recirculation in the vascular access.
In particular, a first known method of measurement in dilution envisages measurement of blood temperature by temperature sensors arranged on the venous line and on the arterial line for monitoring the variation of the respective temperatures in response to a quantity of heat (tracer) administered to or extracted from the blood by means of the dialysis machine.
A second known method of measurement in dilution is described in U.S. Pat. No. 5,312,550 and envisages injection, in the venous line, of a material possessing physical properties different from those of the blood and detection of the condition of recirculation in the vascular access by monitoring the presence of the physical properties of the material upstream from the point of injection of the material.
A third known method of measurement in dilution is described in U.S. Pat. No. 5,510,717 and envisages measurement of conductivity of the blood using a bolus of hypertonic solution injected in the venous line as “tracer” and two conductivity sensors provided on the venous line and on the arterial line for monitoring the variations of the respective conductivity in response to the aforesaid bolus.
A fourth known method of measurement in dilution envisages measurement of blood density using a bolus of isotonic physiological solution injected in the venous line as tracer and two conductivity sensors provided on the venous line and on the arterial line for monitoring the variation of the respective blood densities in response to the aforesaid bolus.
A fifth known method of measurement in dilution envisages measurement of optical absorbency of the blood using a single sensor of dilution of the blood (haematocrit measuring device) provided on the arterial line and, as tracer, a bolus of isotonic solution injected upstream from the sensor; the value AR of recirculation in the vascular access is found by comparing the signal detected by the sensor immediately after injection of the bolus and that observed after the bolus has entered the arterial access.
The provocative and non-provocative methods described above have some drawbacks, however, which have not permitted sufficient and complete exploitation of their merits. In particular, the non-provocative methods are difficult to execute as they require blood samples and laboratory tests, while the provocative methods, as well as being of the invasive type, are imprecise in time and breadth as they require manual injections and can in certain situations be altered by external effects.
Another drawback that is common to all the provocative methods described above is that determination of recirculation in the vascular access is effected by inducing a disturbance in the patient's blood that is of a finite extent and duration, and then calculating the value of recirculation only when the system is in a steady state, i.e. only after the disturbance induced in the patient's blood has passed through the patient's body and has been able to generate an induced disturbance at the point where the measurement sensors are located.
This characteristic that is common to the known methods on the one hand does not permit the execution of continuous monitoring of recirculation in the vascular access but only monitoring at discrete intervals of time that are spaced relatively far apart, and on the other hand does not permit reliable monitoring of the efficiency of the dialysis filter and timely intervention in situations of malfunction of the dialysis machine.
Moreover, in order to obtain an acceptable overall accuracy of measurement of recirculation in the vascular access it is necessary for the signals generated by the measurement sensors to be of sufficient amplitude to provide signal/noise ratios such as guarantee attainment of the accuracy. However, the finite duration of the induced disturbance has the effect that to obtain such amplitudes it is necessary to induce disturbances in the patient's blood that have relatively high amplitudes, which might cause undesirable changes in the dialysis treatment that could, in some particular clinical conditions, be harmful to the organism.
To remedy some of the drawbacks inherent to the known methods, Italian patent IT 1288767 describes a method of measurement that envisages using a provocative measurement performed by varying the ultrafiltration flow in the dialysis filter, without administering a tracer. In detail, the method envisages controlling the variation of ultrafiltration flow of the blood flowing inside the dialysis filter in the course of the dialysis session so as to produce a variation in the concentration of haemoglobin in the blood of a finite extent and duration, then measuring, by means of a haemoglobinometer, the variation in haemoglobin concentration that is induced upstream from the dialysis filter after the blood with altered haemoglobin concentration has been returned to the patient's body, and determining the value of recirculation in the vascular access on the basis of the measured change in haemoglobin concentration.
The method that is the object of the Italian patent cited above, though being particularly advantageous both from the standpoint of simplicity of implementation, as it does not require modifications of the dialysis machine and, using just one sensor, has greatly reduced measurement errors, and from the standpoint of simplicity of execution because, in addition to being of the non-invasive type, can be executed completely automatically without requiring the external interventions of an operator for the administration of a bolus of physiological solutions, still suffers the limitations resulting from determination of recirculation carried out in stationary conditions.